Method of fabricating a steel-filament reinforced metal composite

ABSTRACT

Moldable pellets consisting essentially of a metal matrix having steel filaments positioned therein substantially in parallel and separated from each other by said metal; the method of producing the pellets, and the process of manufacturing shaped composite structures wherein said pellets are molded to the desired shape, e.g., by extrusion or transfer molding.

United States Patent Schierding et al. [4 Aug. 15, 1972 METHOD OF FABRICATING A STEEL- l R r n fi Cited FILAMENT REINFORCED METAL UNlTED STATES PATENTS COMPOSITE 2,953,849 9/1960 Morgan ..29/419 G 72 lnvamors; Royce Schlerdln, 50 San Juan 3,455,662 7/1969 Alexander et al. ..29/ 191.4

St. Charles, Mo. 63301; Tommy L. l Tolhert, s39 Trail Ridge Drive, Rt. ""K' W- 4 cheserfield Mo 63017 ASSISIGII! Examiner-Donald C. Reiley, 111

Attorney-Mary B. Moshier, Thomas B. Leslie and [22] Filed: Jan. 2, 1970 John D. Upham [21] Appl. No.: 405 [57] ABSTRACT Moldable pellets consisting essentially of a metal [52] US. Cl. ..29/419, 29/19l.42,929/;1O.45 matrix having Steel filaments positioned therein sub stantially in parallel and separated from each other by Ila. Cl. ..B 1 i metal; the method f p i g the p and [58] Field of Search..29l4l9 R, 419 G, 191.4, 192 R, the process of manufacturing shaped composite sum} 29/ 187.5, 195 R, DIG. 47, 420.5

tures wherein said pellets are molded to the desired shape, e.g., by extrusion or transfer molding.

3 Claims, No Drawings METHOD OF FABRICATING A STEEL-FRAMENT REINFORCED METAL COMPOSITE The invention described herein was made in the course of or under a contract or subcontract thereunder with the U. S. Department of Defense, Office of Naval Research.

BACKGROUND OF THE INVENTION 1. Field of the Invention Composite structures having a metal matrix and discontinuous, steel fibers as reinforcement.

2. Background of the Invention Use of refractory, discontinuous fibers as reinforcing agents in composites comprising a metal matrix is well known. Although it has been appreciated that parallel orientation of the fibers with respect to each other in a metal matrix contributes to high strength characteristics of the resulting composite, the means of realizing such orientation have been either cumbersome or ineffective. A frequently employed method has consisted of hand-packing the fibers in close proximity to each other in such a manner that the longitudinal axes of the fibers are parallel to each other, and then infiltrating the packing with a melt of the metal which is to serve as matrix. Hand-packing for orientation has many obvious disadvantages. Another method involves hand lay-up of the fibers in a molding with alternating layers of the powdered metal, and molding of the resulting structure by pressing and/or sintering. Whether the matrix metal be admixed with the reinforcing fiber in molten form or in dry, powder form, it is virtually impossible to keep the fibers from sliding and concentrating into pits or crevices of the mold, even before pressure is applied. Upon application of pressure, of course, the loose fibers readily become disoriented. Still another method of orienting reinforcing fibers has involved exposing the fibers, while embedded in a mobile medium, to a magnetic field. Such a method does not provide the versatility required for making various shapes.

Steel fibers or filaments are well known in the art to be valuable reinforcing agents for either organic or metal matrices. Molding of compositions comprising such agents has been generally complicated by their propensity to flex during molding so that parallel orientation, even with the continuous filaments has often been difficult to obtain. Discontinuous, short fibers of steel become very mobile during molding; thus there is not only evidenced difficulty in orientation, but also a tendency for plugging up gates and constricted channels of transfer molding and extrusion equipment. Particularly when the matrix is a metal, mobility of fibers, even during conventional hot-molding compression processes, results in unsatisfactory fiber to metal adherence.

These and other problems relating to utilization of short steel fiber in composites having a metal matrix are solved by the present invention wherein there is provided a method which not only obviates the necessity of hand lay-up for obtaining parallel orientation, but also assures retention of the oriented positioning of the fibers during processing and preservation of the intrinsically excellent reinforcing property of the steel fibers and also provides for very good adhesion of fiber to metal matrix.

SUMMARY or THE INVENTION The invention provides moldable pellets consisting essentially of a metal matrix having steel fibers positioned therein in parallel, said fibers being substantially separated from each other by the matrix metal. The invention also provides the method of producing said pellets and the method of manufacturing shaped composite structures wherein said pellets are molded to the desired shape using heat and pressure.

The reinforcing fiber may be of any type steel. The metal matrix of the pellets is a comparatively low melting metal, e.g., copper, aluminum, zinc, magnesium, silver, indium or any metal, generally, of Groups I to IV of the Periodic Arrangement of Elements. Two or more difierent such metals and or alloys thereof may comprise the matrix.

The pellets are preferably made by coating individual steel filaments with the matrix metal, bonding a plurality of the coated filaments together using the same or a different matrix metal to fonn an elongated body, e.g., a rod, strand, ribbon or tape, and cutting the body into very short lengths, e.g., to form, say, one thirty-second to one-half inch lengths. The elongated body may also be made by infiltration with the molten matrix metal of an assembly of the filaments within a removable housing or upon a support whereby a fixed parallel arrangement of the filaments is made possible.

The pellets are useful molding compounds in the usual molding techniques, e.g., in extrusion, transfer, and compression molding processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention the manufacture of shaped composites comprising a metal matrix and discontinuous steel fiber oriented therein is facilitated by providing pellets of a molding composition wherein each of a plurality of the fibers is surrounded by said metal and bonded together substantially in parallel through said metal. Said pellets are prepared by cutting into small segments an elongated body, e.g., a rod, ribbon or tape consisting essentially of a multiplicity of continuous lengths of steel filament positioned in parallel within the metal matrix throughout the length of the body. By elongated body is meant any unit having a length which is, say, at least ten times as much as any other dimension of the body.

The elongated body which is cut into the moldable pellets is produced by bonding together by means of said metal a plurality of continuous steel filaments in parallel arrangement. Advantageously, this is effected by passing the individual filaments through molten metal to coat them, combining a plurality of the coated filaments into a bundle as they emerge from the molten bath, and allowing the bundle to cool. Depending upon the combining means, the bundle may have a circular or rectangular cross-section, i.e., it may be a rod or strand or a ribbon or tape. Thus when the emerging coated filaments are passed through a circular aperture or a very narrow groove, they will be gathered together into a rod; on the other hand, when they are caused to pass through a narrow slit, they will form a ribbon or tape. Other means of forming the elongated body include separate coating and bundling steps. Thus the individual filament is first coated, either by passing through molten metal and cooling, or by electroplating, or by plasma spraying, or by electrodeless plating, or by vapor deposition. Then the coated filaments are formed into an integral body by passing a plurality of them in contiguous relationship through heated rolls to form a tape or by forcing them together at the softening temperature of the coating metal through a constricted orifice to form a rod or ribbon. The elongated body may also be formed, of course, by infiltrating with the molten metal a fixed, parallel arrangement of large lengths of the filament within a housing or container. The bundles of filaments may also be made by co-extrusion or pultrusion. An easy laboratory method involves simply hand puddling of steel fiber in the molten metal to form a strand. For making the moldable pellets there may be used any elongated body consisting essentially of metal matrix having occluded therein the steel filament oriented in parallel along the length of the body; however, in order that each filament is substantially separated from each other by matrix metal, it is preferred to coat the individual filaments before bundling. Gathering the filaments while the coating is still hot enough to permit adhesion is simply a matter of convenience.

As is known in the art, the coating of a metal unit by treating it with another metal in molten form generally requires that the surface of the unit be very clean in order to obtain an adherent coating. Cleaning of the steel filament prior to passing it through the molten metal, when such step is deemed to be advisable can be effected by any conventional means; however, we have found that treatment of the filament, prior to use, is advantageously effected by passing it through a solution of an acidic salt or a mixture of such salts either in water or preferably, in an organic solvent such as a lower alcohol, whereby cleaning, including degreasing, is achieved. An alcohol, e.g., methanol or isopropanol solution of, say, a mixture of zinc or tin chloride or ammonium chloride or nitrate serves not only as a cleaning means but also appears to activate the steel surface so that very good metal to metal adhesion is thereby obtained.

We have also found that while satisfactory coating of fore it reaches the metal melt, there is obtained cleaning and/or activation of the metal previous to coating. Subsequent travel of the coated filament through the flux does not appear to hurt the coating; on the contrary, thereby better uniformity of coating appears to be demonstrated.

The nature of the salt or salts will depend, of course, upon the melting point of the matrix metal since both the metal and the salts are in the molten state within the bath. Generally, the salts may be the halides, nitrates or sulfates of any metal so long as they melt without substantial decomposition at or above the melting point of the matrix metal.

The thickness of metal coating upon the filament may vary widely, depending upon the filament to metal ratio desired in the final composite structure as well as upon the means of manufacturing the elongated body. When the latter is made by simply passing the individual filaments through molten metal and passing the coated filaments, before the metal coating has hardened, through a single orifice to form a bundle, the thickness of the coating will necessarily determine the filament/metal ratio. Of course, a quantity of metal must be present on the filament such that when several coated filaments are bundled together the filaments adhere to each other through the matrix metal. A substantially uniform coating on the metal is recommended in order to guard against voids and interstices in the bundle; however, uniformity of coating thickness is not critical, because subsequent pressing for production of the final product results in substantial homogeneity. Usually a substantially uniform coating of say, about 0.5 mil in thickness or even less, depending upon the fiber diameter, will suffice for adherence of the fibers to each other within the bundle. However, the coating thickness must be such as to give the desired filler/matrix ratio in the composite body formed by bundling the coated filaments together to cause cohesion of the semi-molten coatings. Otherwise, infiltration or other techniques which permit introduction of additional metal must be used to form the rod-like bundies.

The thickness of the elongated body will vary with the geometry of its cross-section. In the case of rods or strands the diameter will vary from, say, about 10 to mils. In the case of ribbons or tapes, the cross-section will have one dimension of from 10 to I00 mils and another dimension which will correspond to the thickness of from 1 to about 5 coated wires, say, a dimension of from about 1 to 20 mils. Generally, pellets cut from elongated bodies of either circular or rectangular cross-section will have a length of from about 0.03 to 0.5 inch and the next greater dimension of from about 0.01 to 0.1 inch with the dimension lengthwise the body being greater than any other dimension of the pellet.

The size of the pellets into which the elongated body is cut will depend somewhat upon the procedure to be used for making the finished composite. For extrusion and transfer or flow molding it is advantageous to cut the rod, ribbon or tape into lengths which will vary in length from, say, 0.03 to 0.5 inch and preferably from about 0.06 to 0.25 inch. The pellets may contain substantially any number of steel filaments. Generally, depending upon the thickness of the filament, there will be from, say, about 5 to about 50 filaments longitudinally disposed parallelwise in the bundle or grouping which makes up the elongated body. Cutting the body into the pellet-size lengths, results, of course, in pellets containing the same number of filaments in the same orientation.

The pellets are useful molding compositions for generally employed molding techniques. Thus, they may be extruded, i.e., forced through a constricted orifice of substantially any geometrical configuration or size to give shaped extrudates having discontinuous steel fibers occluded therein in substantially parallel, overlapping array. Separation of the fibers from each other in the pellet-size molding compound prevents the fiber from being crushed or grossly deformed, which may occur when the fibers are randomly positioned as, for instance, in a molding mix of the dry components or in fragments of preformed mats formed by mixing short fiber with the molten metal and cooling. The presently provided pellets are likewise useful in conventional transfer molding and plunger molding processes as well as in conventional compression molding. The small pellets may actually be considered to be preforms; hence, they are particularly useful in transfer and flow molding processes in that they provide for reduced bulk factor and uniform density, thus assuring uniform preheating and controlled mobility of the molding compound from the receptacle through the gate. Alignment of the fibers in the pellets serves to reduce breakage thereof, and such alignment is retained in the finished molding.

Flow molding or extrusion of the pellets may be conducted at even below the melting point of the matrix metal without substantial fiber breakage. lndeed, use of temperatures below the melting point of the matrix is beneficial in that it permits controlled flow rate and assures that fiber and matrix flow together. Such controlled flow from the heated receptacle through the similarly heated runners and gates and into the forming die thereby permits controlled fiber orientation and distribution, so that either rod or sheet of unidirectionally oriented, overlapped fibers or complex, contoured shapes having locally controlled fiber orientation can be produced.

Advantageously, the temperature employed for either extrusion, transfer or flow molding or compression molding is below the melting point of the pure metal, or below the liquidus temperature of the alloy (for control of fiber flow). Also at such temperatures the possibility of any deleterious fiber-matrix interaction is minimized. However, the temperature should be high enough to assure flow of the molding compound during molding. Preferably, the pellets should be heated in the receptacle to a temperature of about from 5 C. to 30 C. below the matrix melting or liquidus temperature. Lower temperatures can be used provided the equipment can deliver enough force or pressure to do the job. Of course, other factors may enter in at lower temperatures such as the surface condition of the molded part. Heating in the receptacle should be conducted for a period which is long enough to insure thorough heating of the molding compound, which period is preferably less than one hour. The molding equipment, e.g., orifice, runner, gate and platen should be maintained at about the same temperature. The pressure used in flow molding or extrusion techniques is determined by several factors, including runner and gate dimensions, matrix and fiber properties, temperature, reduction ratio, lubrication, etc. The pressure conditions to be used with a particular molding composition and with a particular technique and equipment is thus commonly and necessarily arrived at by routine experimentation which is well within the purview of those skilled in the art.

Substantially any metal may be used as the matrix; however, because the readily softened metals are more useful in conventional hot molding processes, the pellets are most advantageously prepared from such metals as, say, aluminum, magnesium, copper, zinc, silver and lead and other metals of, e.g., Groups I to IV of the Periodic Arrangement of Elements. Alloys of such metals are generally useful. Instead of a single metal, two or more may form a matrix. Thus, the long steel filament may be coated first with one metal and then with one or more successive coats of another metal previous to bundling. Also, filaments which have been metal coated may be assembled parallel to each other within a container and the resulting assembly infiltrated with another metal. In the present instance, there appears to be no degradation of the steel fiber or filament during fabrication of the pellets or of the composite structures molded therefrom; and, although interaction of a kind may occur between steel and the matrix metal, such interaction if any, results in a beneficial effect, possibly owing to formation of some metal steel linkages between filament and matrix which results in enhancement of mechanical and thermal properties.

The reinforcing fiber may be of any of the iron alloys commonly known as steels, e.g., vanadium steel, molybdenum steel, stainless steel, etc., in filamentous form. The thickness or cross-sectional dimension of the steel fiber may also vary over wide limits; but, in general in the case of the round fibers, those having diameters between about 0.0005 inch to 0.025 inch are preferred. in the case of rectangular fibers, those having cross-sectional areas equivalent to those of the aforementioned round fibers are presently preferred. When the fibers are used in discontinuous form, i.e., in short lengths, the aspect ratio (length/diameter) will advantageously be from about 25 to 1,000. If desired, the steel fibers may be used in the form of roving wherein the individual fibers may be of the order of, say, 3 to 15 microns diameter.

The amount of steel filament in the molding pellets and hence in the shaped composites molded therefrom will vary greatly depending upon the properties desired; however, in order to impart significant improvements as compared to the un-reinforced metal, the filament should be present in a quantity of at least 5 per cent by volume of the pellet or molding. Steel filament loadings of as high as about 90 per cent by volume are attainable; but, to obtain optimum modulus and strength characteristics, it is preferred to employ the steel filament in a quantity which is from, say, about 15 per cent to about 60 per cent by volume of the pellet or finished composite.

For processing convenience in extrusion or transfer molding, it may be desirable to introduce the fibrous reinforcing agent in the form of very highly loaded pellets containing from 40 to per cent of fiber and subsequently, during processing, to blend these pellets with granules or a melt of pure metal to achieve the finally desired loading of reinforcing agent uniformly distributed and oriented in the molding.

The filament is present in the elongated body and in the pellets in continuous form; in the molded structures, it is present in discontinuous form. By continuous form" is meant the positioning of the filament along one dimension of the reinforced unit. By discontinuous form is meant use of very small lengths of the filament, say, pieces which may vary from about one thirty-second inch or less to about one-fourth inch, which pieces are smaller than any one dimension of the reinforced unit. Presence of continuous lengths of the filament in the elongated body and in the pellet generally provides for uniaxial positioning of the filament; the orientation thus obtained, plus the presence of matrix metal between the filaments eliminates contact of the filaments with each other so that during molding there is eliminated the filament damage which occurs when filaments are forced against each other during pressing.

The invention thus provides a new and valuable method by which a metal/steel fiber mixture can be flow molded or pressure formed into complex three dimensional shapes or extruded without extensive fiber breakage, thereby retaining the high reinforcement efficiency of steel fiber.

The invention is further illustrated by, but not limited to, the following examples:

ELE 1 This example shows the manufacture and molding of pellets consisting of zinc as the matrix metal and stainless steel roving (Brunswick Stainless Steel MF-Al roving), Said roving consisting of 12 micron diameter fibers. Steel roving was first cleaned by passing through a solution of a 221i weight ratio mixture of zinc chloride and ammonium chloride in methanol, then pulled through a bath of molten zinc of standard galvanizing grade (P.W. slab, American Zinc Co.), at 450 C. at a rate of 100 ftJminute, and finally collected on a reel. Microscopic examination showed that the roving fibers had been coated by the zinc. Subsequently, lengths of the reeled material were cut into 54;. inch grains and the grains were extruded at 400 C. and 400 lbs. oil pressure through a one-fourth inch, molybdenum sulfidecoated steel orifice. The extruded, smooth-surfaced rod possessed a high degree of essentially unbroken fiber collimation along the sample axis.

The extrudate, having a fiber loading of about 20 volume per cent had a flexural strength of 41,700 psi, as compared to the tie strength value of 28,200 psi for an unfilled zinc rod prepared by the same procedure.

EXAMPLE 2 This example shows the manufacture and molding of pellets consisting of zinc as the matrix metal and steel music wire (0.003 inch diameter, Johnson Steel and Wire Co., Worcester, Mass).

A graphite block was clamped on each side of the ceramic cover plate of a hot plate. The steel wire was individually threaded through six small separate grooves in the first graphite block, passed through a pool consisting of a flux (a 1:1 weight mixture of zinc chloride and ammonium chloride) and molten zinc on the hot plate, and then converged through a single groove in the second graphite block to form a bundle. The steel filaments became completely coated with zinc during their continuous passage through the pool. There resulted a uniformly coated steel bundle wherein each zinc-coated filament of the bundle was bonded to another filament through the layer of zinc. The zinccoated steel bundle was taken up on a reel as formed. Subsequently, lengths of the reeled material were cut into three-sixteenth inch length grains and the resulting pellets were extruded at 400 C. from a 1 inch extrusion chamber through a three-sixteenth inch diameter tapered (4:1 area reduction) orifice. The fibers in the extruded rod thus obtained were well collimated and essentially unbroken.

Transfer molding of the pellets at about 400 C. by forcing them under pressure through a runner and gate heated at the same temperature into a heated mold gave standard tensile specimens exhibiting similarly improved strength.

EXAMPLE 3 Using the procedure of Example 2, the steel wire described in that example was passed through a bath consisting of molten aluminum floating on a flux composed of barium chloride, sodium chloride, and aluminum fluoride in a :20:10 weight ratio. The bundle of six aluminum-coated wires thus obtained was cut into one-fourth inch pellets and extruded at a temperature of about 25 C. below the melting point of aluminum to give a smooth elongated body having a diameter of about five-sixteenth inch. Etching of a cross-section of the extrudate with dilute hydrochloric acid and microscopic examination of the etched surface showed parallel orientation of the fiber and no fiber damage.

EXAMPLE 4 Using the procedure of Example 2, ten stainless steel wires having a diameter of 0.005 inch were individually passed through a pool of tin on the ceramic cover of a hot plate. The coated bundle of ten uniformly spaced and bonded wires thus obtained was cut into one-eighth inch long pellets, and the pellets were extruded at a temperature of about 220 C. through a one-eighth inch by one-half inch rectangular orifice to give a smooth tape in which the discontinuous stainless steel reinforcement was present as a parallel, overlapping array aligned axially along the length of the tape. Segments of this tape were then aligned side-by-side, five layers deep, with alternate layers off-set by one half tape width, in a 2 X 4 inch rectangular positive-pressure mold and bonded together under pressure to yield a sound, void-free 2 X 4 inch, unidirectionally reinforced plate.

Operating as above described steel filament is readily incorporated into other metal matrices, e.g., magnesium, copper, silver, gallium, titanium, etc., to provide molding compounds for speedy, potentially low cost forming of steel-reinforced metal structures. The presently provided molding compounds of pre-encapsulated, collimated steel fibers thus constitute a notable improvement in the art.

Employing the pellets and the fabrication method made possible thereby, there are readily produced extremely tough, shaped composite structures which, depending upon the configuration of the mold, are useful in numerous industrial and space applications wherein high-strength components are required; e.g., bearings, gaskets and machine tools of all kinds.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention.

What we claim is:

l. The method of manufacturing a shaped composite structure wherein moldable pellets consisting essentially of a. a metal of Groups I to IV of the Periodic Arrangement of Elements and b. steel filaments positioned substantially in parallel in the metal and separated from one another by said metal, are extruded through a restricted orifice at a temperature which is below the melting point of the matrix metal but at a temperature which is sufiicient to effect flow of the metal at the pressure of extrusion. 

2. The method defined in claim 1 wherein said moldable pellets are transfer molded by charging the pellets to a receptacle; heating the pellets to a temperature below the melting point of the matrix metal but sufficiently high to permit flow of the pellets; and applying pressure to force the pellets to flow through a runner and gate into a mold while maintaining the pellets at said temperature.
 3. A method according to claim 1 wherein said steel filaments comprise from about 15 to about 60 percent by volume of said moldable pellets. 